Atmospheric oxidation in the presence of clouds during the Deep Convective Clouds and Chemistry (DC3) study

Abstract

Abstract. Deep convective clouds are critically important to the distribution of atmospheric constituents throughout the troposphere but are difficult environments to study. The Deep Convective Clouds and Chemistry (DC3) study in 2012 provided the environment, platforms, and instrumentation to test oxidation chemistry around deep convective clouds and their impacts downwind. Measurements on the NASA DC-8 aircraft included those of the radicals hydroxyl (OH) and hydroperoxyl (HO2), OH reactivity, and more than 100 other chemical species and atmospheric properties. OH, HO2, and OH reactivity were compared to photochemical models, some with and some without simplified heterogeneous chemistry, to test the understanding of atmospheric oxidation as encoded in the model. In general, the agreement between the observed and modeled OH, HO2, and OH reactivity was within the combined uncertainties for the model without heterogeneous chemistry and the model including heterogeneous chemistry with small OH and HO2 uptake consistent with laboratory studies. This agreement is generally independent of the altitude, ozone photolysis rate, nitric oxide and ozone abundances, modeled OH reactivity, and aerosol and ice surface area. For a sunrise to midday flight downwind of a nighttime mesoscale convective system, the observed ozone increase is consistent with the calculated ozone production rate. Even with some observed-to-modeled discrepancies, these results provide evidence that a current measurement-constrained photochemical model can simulate observed atmospheric oxidation processes to within combined uncertainties, even around convective clouds. For this DC3 study, reduction in the combined uncertainties would be needed to confidently unmask errors or omissions in the model chemical mechanism.

Introduction to the Deep Convective Clouds and Chemistry (DC3) 2012 Studies

“…The 21 June 2012 DC3 case of a decaying MCS provided measurements of the chemical composition of the dissipating anvil region and convective outflow of the MCS over an 11-hr period during daytime. During the morning period (the first 5 hr of measurements), ozone increased by~12 ppbv (Brune et al, 2018), consistent with observed HO 2 and box model calculations, and another 8 ppbv (20 ppbv in total) by midafternoon . Results from these two case studies are in the upper end of the range reported by previous studies.…”

“…Using the WRF‐Chem model at convective parameterization scales, Li et al () found that wet scavenging overestimated removal of these soluble trace gases, but the predicted scavenging efficiency improved compared to observations when ice retention fractions for each trace gas were included in the parameterization. In general, observations in the upper troposphere outflow regions found that formaldehyde had the highest mixing ratios of these three HO x precursors and was calculated to be the dominant HO x precursor for ozone formation downwind of the storms (Brune et al, ).…”

On the sources and sinks of atmospheric VOCs: An integrated analysis of recent aircraft campaigns over North America

“…In contrast to the PBL where both positive and negative model discrepancies occur, aloft in the FT the model exhibits a large negative bias for both VOC-carbon (-64%) and reactivity (-63%) that manifests essentially everywhere. Such a severe discrepancy has implications for our understanding of FT HOx cycling (Brune et al, 2018;Mao et al, 2009), ozone production at higher altitudes where its climatic effects are strongest (Apel et al, 2015;Bertram et al, 2007), and possibly, secondary organic aerosol loading (Bianchi et al, 2016;Cappa, 2016;Kirkby et al, 2016;Trostl et al, 2016;Heald et al, 2005). We explore potential causes for these observed discrepancies in Sec 6.1.…”

On the sources and sinks of atmospheric VOCs: an integrated analysis of recent aircraft campaigns over North America

“…In contrast to the PBL where both positive and negative model discrepancies occur, aloft in the FT the model exhibits a large negative bias for both VOC carbon (−64 %) and reactivity (−63 %) that manifests essentially everywhere. Such a severe discrepancy has implications for our understanding of FT HO x cycling (Brune et al, 2018;Mao et al, 2009), ozone production at higher altitudes where its climatic effects are strongest (Apel et al, 2015;Bertram et al, 2007), and possibly secondary organic aerosol loading Cappa, 2016;Kirkby et al, 2016;Heald et al, 2005). We explore potential causes for these observed discrepancies in Sect.…”